The present invention relates to a magnetic field generator, which generates a magnetic field, and more particular, to a magnetic field generator, which uses a superconducting magnet.
A superconducting magnet is used in MRI (Magnetic Resonance Imaging) apparatuses. The superconducting magnet is kept at extremely low temperature by liquid helium. The liquid helium is always cooled to temperatures equal to, or lower than its evaporating temperature.
MRI apparatuses are constructed so as to normally function even when power goes down. Backup power supplies are provided in hospitals against power failure. Further, even when a refrigerator stops, the heat capacity of the liquid helium inhibits temperature rise of a superconducting magnet. Accordingly, even when a refrigerator stops due to power failure, it is possible to maintain the superconducting magnet in a superconductive state for about two or three days or more.
JP-A-2005-116956 discloses an open type MRI apparatus, which uses a superconducting coil (superconducting magnet). The MRI apparatus is constructed so that a liquid helium vessel is surrounded by a heat shield, which is further surrounded by a vacuum container.
In recent years, high-temperature superconducting materials are developed, and therefore, it has become to make an electromagnetic coil from a high-temperature superconducting wire material. Since the high-temperature superconducting material is higher in critical temperature than metallic superconducting materials such as NbTi, etc., a superconductive state can be held by cooling with liquid helium, or direct cooling with a refrigerator. Further, the high-temperature superconducting material has an advantage that it is unnecessary to use a liquid helium, which is expensive and difficult to handle. With a superconducting magnet, however, the lower temperature becomes, the higher critical current value can be obtained. Therefore, a demand for utilization of a lower temperature than 77 K being a temperature of liquid nitrogen is increased.
JP-A-2002-208512 discloses a cooling construction making use of a high-temperature superconducting coil (superconducting magnet). With the cooling construction, the high-temperature superconducting coil (superconducting magnet) is cooled directly by a refrigerator and cold generated by the refrigerator is made use of to generate solid nitrogen. With the example described in JP-A-2002-208512, the solid nitrogen is made use of to inhibit temperature rise of the high-temperature superconducting coil when a refrigerator stops. Since the solid nitrogen has a large specific heat per weight as compared with other metals, etc., it is possible to make a whole apparatus lightweight.
With a MRI apparatus, which uses a superconducting magnet (superconducting coil), it is necessary to generate an intense magnetic field at a patient's position. With, for example, the open type MRI apparatus described in JP-A-2005-116956, it is preferable that a distance between upper and lower superconducting magnets is smaller. However, it is required that a sufficiently large space to arrange a patient be provided between the upper and lower superconducting magnets. Accordingly, it is not possible to make a distance between the upper and lower superconducting magnets smaller than a predetermined dimension.
Further, the construction shown in JP-A-2002-208512 involves a possibility that when a refrigerator stops due to power failure or malfunction, the superconducting magnet (superconducting coil) is increased in temperature by heat, which flows back from the refrigerator itself.
It is an object of the invention to provide a magnetic field generator capable of presenting an intense magnetic field in a position of use and further maintaining a superconductive state over a long term even when a refrigerator stops due to power failure, etc.